272* METALLURGY OP IRON A1STD STEEL. angles.—It is very difficult to make a comparison of two different structural shapes, since it does not often happen that the same heat is rolled into more than one kind of section, but an attempt is made to do this in Table XIV-M. The prime requisite is that the steel in both cases shall be of the same manufacture, and this specification is satisfied in the present instance. The figures for the angles are found by combining certain groups in Table XIV-H, which was compiled from the records of The Pennsylvania Steel Company, while the plates represent the average obtained from The Paxton Rolling Mill, which was running on slabs from the same works. The 9ne predominant feature is the lower elongation in the plates. This may be explained by the fact that the metal receives a less thorough compression in the plate train than it does in the rolling of angles, in which latter case it undergoes a certain amount of lateral thrust. SEO, XlVk.—Effect of thickness on the physical properties, of plates.—The effects : caused by variations in rolling temperature appear in their most marked degree in the comparison of plates of different; gauges. It is not customary to test the same heat in several sizes, but by long experience the manufacturer is able to judge the relative properties of each thickness. The heads of two . widely-known plate mills .have- given me as their estimate that, taking one-half inch as a basis, there will be the following changes in the physical properties for every increase of one-quarter inch in thickness: . • (1) A decrease in ultimate strength of 1000 pounds per square inch. (2) A decrease in elongation of one per cent, when measured in an 8-incIi parallel section. • . > (3) A decrease in reduction of area of two per cent. W> E. Webster* gives the same data on ultimate strength, but does not mention the relation of section to elongation. . >It is, therefore, plain that in the writing of specifications some allowance must be made for these conditions, since a requirement which is perfectly proper for a three-eighth-inch plate: will be unreasonable for a 1%-inch. Moreover, the effect is cumulative, since a harder steel must be used in making the thick plate and * Observations on the Relations "between the Chemical Constitution and TJW-foate Strength of Steel. Journal I. and 8,1., Vol. I, 1894, p. 329.